1. Technical Field
The present invention relates to a vibration power generator and a vibration power generating device, and a communication device and an electronic device having the vibration power generating device mounted thereon, and more particularly to an electrostatic induction vibration power generator and a vibration power generating device using an electret material, and a communication device and an electronic device having the vibration power generating device mounted thereon.
2. Description of the Related Art
There has been known, as a conventional vibration power generating device, a static induction vibration power generating device in which electric charges are provided to one electrode of a variable capacitance, and charges are induced to an opposed electrode by a static induction. A change in the induced electric charges is brought about by a change in electric capacity. According to the static induction vibration power generating device, electrical generation is performed by extracting the change in the electric charges as electrical energy (refer to, for example, Patent Document 1).
FIG. 24 is a schematic cross-sectional view showing a vibration power generator 10 described in Patent Document 1 (refer to FIG. 4 and pages 10 to 11 of the document) as an example of a conventional electrostatic induction vibration power generator using an electret material.
The vibration power generator 10 is composed of a first substrate 11 provided with a plurality of conductive surface regions 13, and a second substrate 16 provided with a plurality of electret material regions 15. The first substrate 11 and the second substrate 16 are arranged and spaced from each other at a predetermined interval. The second substrate 16 including electret material regions 15 is fixed. The first substrate 11 including conductive surface regions 13 is coupled to a fixation structure 17 through springs 19. The springs 19 are connected to both side surfaces of the first substrate 11, and also connected to the fixation structure 17. The first substrate 11 is capable of returning to its home position due to the springs 19, or the first substrate 11 makes a lateral motion (for example, a motion in a crosswise direction of the drawing) to be capable of returning to the home position. This movement brings about increase and decrease of the overlapping area between the electret material regions 15 and the opposed conductive surface regions 13, which results in a change of electric charges in the conductive surface regions 13. A vibration power generator (electrostatic induction vibration power generator) 10 performs electrical generation by extracting the change of electric charges as electrical energy.
Then, according to frequency of vibration used in the electrical generation, a resonance frequency of vibration of the first substrate 11 is selected.
However, in the vibration power generator 10, since the resonance frequency is determined depending on the first substrate 11 and the springs 19, there is a problem that it is difficult to reduce a resonance frequency as described hereinafter.
In order to reduce the resonance frequency, it is necessary to increase the mass of the first substrate 11 or decrease a spring constant of the springs 19. The springs 19 are usually formed of silicon or the like, and it is difficult to decrease the spring constant because of an elastic constant of the material, or the size of the spring. Therefore, it is necessary to increase the mass of the first substrate 11.
However, when the resonance frequency is reduced by increasing the mass of the first substrate 11, a large force is applied to the springs 19 by vibration of the first substrate 11 (larger strain is generated by the springs 19), thus causing a problem such as deterioration of durability of the springs 19.
In order to solve this problem, there is proposed an electrostatic induction vibration power generator using resin springs that is excellent in resistance to elastic strain and enables reduction of a resonance frequency (refer to, for example, Non-Patent Document 1).
FIG. 25 is a schematic perspective view showing a conventional vibration power generator (electrostatic induction vibration power generator) 20 using resin springs described in Non-Patent Document 1
The vibration power generator 20 is composed of a first substrate 21 with elecrets 29 formed thereon, and springs 29 made of a resin that connect the first substrate 21 to a fixation structure 27. The springs 29 is constituted using a parylene resin material having durability such as fatigue resistance, and therefore enables the first substrate 21 to vibrate at a comparatively low frequency and a large amplitude.
Furthermore, since the spring 29 has a high-aspect-ratio structure in which a ratio of the length in a width direction (y-axis direction in the drawing) and a thickness direction (z-axis direction in the drawing) of the first substrate 21 to the length in a length direction (x-axis direction in the drawing) is large, the first substrate 21 undergoes vibration (forced vibration) only in a uniaxial direction without causing vibration of the spring 29 in the width direction (y-axis direction) and the thickness direction (z-axis direction) of the substrate.